The invention relates to a diode laser subelement with at least one laser diode element applied to a mounting surface of a heat-distributing multilayer substrate, whereby the multilayer substrate is joined materially with one another, made of an upper, a lower, and at least one separating layer interposed therebetween.
Laser diode elements include individual laser diodes and laser diode bars. Individual laser diodes have only a single laser diode emitter that is defined by an optically continuous active area. Laser diode bars are monolithic semiconductor laser arrangements of at least two operational laser diode emitters that are optically largely independent of one another. The lateral reach of laser diode bars is a factor of the width, spacing, and number of emitters. Typical widths are in the range of 1 to 15 mm.
For reliable operation as a diode laser, the laser diode elements are mounted on a heatsink that distributes, and, in the case of microchannel coolers, also carries off, heat loss. The upper side of the heatsink is defined in that it contains the surface for assembling the laser diode element.
It is known that two essential conditions must be observed for reliable solder mounting of laser diode bars on heatsinks: (a) the use of hard soldering or performing diffusion soldering with isothermal solidification and (b) the use of a heatsink, the thermal expansion coefficient of which corresponds to that of the laser diode material to approximately 1 ppm/K in order to attain minimum stress soldering of the laser diode bar, which is relatively wide in comparison to the individual laser diodes.
In particular for high-performance laser diodes with a GaAs basis (i.e., the epitaxy of which is performed on a GaAs substrate), special heatsink embodiments must be found because the highly heat-conductive materials that are suitable for cooling either have thermal expansion coefficients that are too low (aluminum nitride, silicon carbide, boron nitride, and diamond are at least 2 ppm/K below the value for GaAs of 6.5 ppm/K) or too high (aluminum, gold, copper, silver are at least 6 ppm/K above the value for GaAs).
The same is true for sapphire-based laser diode bars with GaN epitaxy and for zinc selenide-based laser diode bars.
Also known is designing heatsinks with high thermal conductivity with a thermal expansion coefficient that corresponds to that of the laser substrate material (GaAs, sapphire, GaP, GaSb, ZnSe) using a multilayer structure with layers made of materials with higher and lower thermal expansion coefficients. One example is the copper/aluminum nitride/copper system. Such heatsinks can also be embodied as microchannel coolers. In the interest of the lowest possible thermal resistance, recesses and cutouts can be added to the individual strata or layers for varying strata thicknesses.
The known technical solutions largely relate to the thermo-mechanical goal of adjusting the appropriate thermal expansion coefficient and to the thermal goal of optimized cooling. However, one important aspect for the operation and engineering of a diode laser is also power supply for the laser diode bar.
The heatsink is often also one (the first) of the two electrical contacts, because the laser diode must carry off the main heat via one of its poles (anode or cathode). A small secondary amount of heat can be carried off the via the second pole; it then becomes critical when the second electrical contact enters a thermally highly conductive connection, for instance with the heatsink, that already provides the first electrical contact.
In the prior art (DE 101 13 943 A1), a metal film is mounted to the secondary heat elimination side of the laser diode bar. The thermal and mechanical link to the generally larger heatsink (electrical contact of the main heat elimination side) occurs with electrical insulation from the same via a thin electrical insulating stratum that is approximately the thickness of the laser diode bar, for instance made of polyamide or ceramic. Thus there are two joint zones situated between the secondary heat elimination side-electrical feed line, above and below the electrical insulation stratum, which means disadvantageous mounting complexity.
Another disadvantage in known diode lasers relates to providing connection options for electrical contact elements for the electrical supply of the laser diode bar via power cable. As a rule, these electrical connection options are realized in the electrical contact elements as threaded bores that permit the power supply cable to be attached via pole pieces using screws. As has been stated, the main heat elimination side-electrical contact element is a component of the heatsink. In accordance with the prior art, the secondary heat elimination side-electrical contact element is brought into electrical contact with the line that is joined to the laser diode bar on the secondary heat elimination side in a material fit. This occurs either directly in a material fit in that the secondary heat elimination side-electrical contact element is attached only to the line that itself obtains its pull relief via the material-fit mounting to the electrical insulation stratum attached to the heatsink, or indirectly in a force-fit in that the secondary heat elimination side-electrical contact element is screwed to the heatsink or to a third element. In both cases a contact or joint zone occurs that involves disadvantageous mounting complexity. In addition, both the electrical contact of the contacts via pole pieces and screws and a force-fit contact of the contact element to the strip conductor contain an electrical contact resistance, the heat of which should be carried off efficiently during operation.
Another unsatisfactorily addressed object relates to the modular integration of diode laser elements or diode lasers in arrangements of a plurality of diode laser elements or diode lasers. There are numerous suggestions for vertical arrangements (stacks) and lateral arrangements (rows) of diode laser elements, both for conductively cooled substrates and for convectively cooled microchannel heatsinks. While series connection of the diode laser elements is realized in a simple manner in these vertical diode laser arrangements in that the strip conductor, attached to the laser diode bar on the secondary heat elimination side, of a first diode laser element is in direct or indirect electrical contact with the heatsink as a main heat elimination side-electrical contact of a second diode laser component located thereabove, for lateral diode laser arrangements there is the problem of converting the vertical electrical contact element arrangement into a series connection of the diode laser elements that can be arranged laterally. This goal of series connection of lateral diode laser arrangements can be generalized for every planar arrangement of diode laser elements and diode laser stacks.